THE SUN

Similar in many languages, "Sun," related to Latin's "sol," was
represented in ancient Greece as Helios, god of the Sun. Giver of
warmth and life, we hardly think of the Sun as a star, the term
"Sun and stars" in constant use, though the surmise that it is a
star goes back to ancient times. It is different because it is OUR
star, the one that belongs to us, the one we can see most closely,
and the one we know most about. (Though AT NO TIME ATTEMPT TO
VIEW THE SUN, as it is so bright it can burn the eye; leave
that to professionals.) With the Sun only 150 million kilometers
(93 million miles) away, astronomers can detect incredible detail.
The next nearest star of similar brightness, Alpha Centauri, is 271,000 times farther, each of
its two components (it is a double star) appearing as mere
points. The Sun is the reference to which all other stars are
compared, their diameters almost always expressed in solar
diameters, their brightnesses in solar luminosities.

To understand the stars better, to make sense of their real
characteristics, we need to know the solar
characteristics which, while the Sun is hardly the brightest
and biggest star in the sky, are still astounding. It is 1.4
million kilometers across, the equivalent of 109 Earths set side by
side, and has a mass of two million trillion trillion kilograms, or
330,000 Earths. Most astonishing perhaps is its luminosity of 400
trillion trillion watts. To put that in perspective, it would cost
the gross national product of the United States for millions of
years for a local power company to run the Sun for one second.
This immense energy, pouring from a body with a yellow-white
"surface" (a highly opaque gas called the photosphere) of
5800 Kelvin, is generated by thermonuclear fusion (of hydrogen into helium) in the Sun's
deep core, where the temperature reaches more than 15 million
Kelvin and the density hits 14 times that of lead. No matter the
density, however, the core, like the rest of the Sun, is entirely
gaseous. Taking up about half the Sun's mass and a quarter of its
radius, the core is surrounded by an inert envelope whose outer
third or so is in a state of roiling convection (hotter gases rising,
cooler ones falling). The outer layers are made of 91.5 percent
hydrogen, 8.5 percent helium (the element named after Helios, since
helium was discovered there first), and a bit over a tenth of a
percent of everything else, oxygen dominating these followed by
carbon, neon, and nitrogen (known from analysis of the solar spectrum). In the heat and
pressure of the core, atoms of hydrogen are slowly being converted
into those of helium (four H into one of He in a three-step
process), a small amount of mass lost and converted to energy in
the process (via Einstein's famed equation E = mc**2, where
c is the velocity of light). After 4.5 billion years (as
found from the ages of meteorites), the core of the Sun is now
about half helium, and there remains enough hydrogen to last for
another five or so billion years.

When the core hydrogen finally runs out, the Sun will temporarily
spike in brightness by up to a thousand times its current
luminosity, expand, and cool at the surface. Under increasing
compression, the helium created earlier within the nuclear furnace
will begin to fuse to carbon and oxygen, causing the future Sun to
dim back some to become a modest red giant star like so many of
those that populate the naked-eye sky. When the helium is gone,
the Sun will brighten even more, to some 5000 times its present
luminosity, expand to nearly the size of Earth's orbit, and become
even cooler and redder. Varying in brightness as an advanced giant (rather like the
star Mira), it will slough off its outer
hydrogen layers, exposing the core. The core in turn will
illuminate the expanding debris to briefly create a planetary nebula (a misnomer having nothing to
do with planets), and will then die and cool as an ultradense,
dimming carbon-oxygen white
dwarf about the size of Earth with somewhat over half the Sun's
current mass (showing that stars finish their lives with a
lotless mass than they start out with).

The Sun spins slowly with a period of 25 days at its equator, the
spin and churning outer gases producing a magnetic field about five
times the strength of Earth's. The rotation wraps the internal
field into powerful ropes that rise upward to break through the
surface, where they chill local areas by inhibiting convection, and
thus create the famed sunspots. Sunspots come in pairs, one
where the field goes out of the solar surface, the other where it
re-enters. Numerous spots commonly gather into packs within
centers of activity, the tangled fields making it difficult,
even impossible, to see which ones belong together. The magnetic
fields are unstable, so the individual spots do not last long, just
days to perhaps a month. The magnetism heats a tenuous outer
layer, the corona, to around two million Kelvin. The
corona's thinness makes it dim and visible to us only during a
total eclipse (when the bright surface is blotted out by the Moon)
or from space. Controlled by magnetism and luminosity, from the
opaque hot corona flows a thin but fast wind that blasts
past the Earth and makes comet tails point away from the Sun.
Collapsing solar magnetic fields produce localized powerful flares
and release coronal gases that fly down the solar wind. If one of
these coronal mass ejections hits Earth, it can massively
disturb its magnetic field to produce the northern and southern
lights (the aurora), can wreck
satellite systems, and have even been know to bring down power
grids on the ground. Solar magnetic activity, seen most vividly in
the number of sunspots, is cyclic, coming and going over an average
of 11 years. At minimum, there may be no spots at all, while at
maximum the Sun can be covered with them. Many of the same
phenomena are seen in the stars around us, the Sun providing a way
to understand them, the stars in turn allowing us better to
understand the Sun, our own personal star.

The following two tables give a summary of solar properties
and a list of stars similar to the Sun, both taken from Stars and their Spectra (J.
B. Kaler, Cambridge University Press, 2011). A primary source for
the first table is Allen's Astrophysical Quantities, 4/ed,
AIP Press/Springer, 1999. Exponents are expressed by "**." The
data in the second table may differ some from those in the
individual stellar essays and in the table of Brightest Stars. Distances are in parsecs; multiply by 3.26 to
get light-years. Where they are given in the Remarks, ages are in
"Gyr," billions of years.